feat: add subentities support

[Sekhmet]

Summary

Closes #166

This PR adds @derivedFrom directive support that can be used to create 1-n queries.

Test plan

Add a field on some entity that references other entity for example:

type Space {
  id: String!
  proposals: [Proposal]! @derivedFrom(field: "space")
}

type Proposal {
  id: String!
  space: Space
}

This will cause proposals to become available as part of Space schema resulting in this query to database (table name is derived from type used, column name is the field argument from directive):

SELECT * FROM proposals WHERE space IN (space.id)

Now such queries can be used:

{
  spaces {
    id
    proposals {
      id
      tx
      title
      strategies
    }
  }
}

You can test it against this branch on sx-api (extra changes due to event parsing): https://github.com/snapshot-labs/sx-api/tree/sekhmet/subentities

[bonustrack]

tACK. If we always return all the entries it may take too long to resolve or become unusable, ideally we should limit the results and allow custom query like root operation example:

{ 
  spaces {
    name
    proposals (first: 10, orderBy: end) {
      title
    }
  }
}

But this can be solved in future, this PR is already a great step.